A gas turbine engine may have a plurality of coaxial spools. For example, an aero gas turbine engine can have a low pressure spool formed by a propulsive fan, low pressure turbine and respective interconnecting shaft, and a high pressure spool formed by a high pressure compressor, high pressure turbine and respective interconnecting shaft, the shaft of the low pressure spool extending through the shaft of the high pressure spool. Some engines also have an intermediate pressure spool formed by an intermediate pressure compressor, intermediate pressure turbine and respective interconnecting shaft. Each fan or compressor is driven by the respective turbine. The compressor(s) forces compressed air into a combustor, where the air is mixed with fuel and ignited to produce a flow of hot combustion gas from which the turbines extract energy.
At particular points in a typical aircraft flight cycle the engine operability and/or efficiency may be improved by the transfer of mechanical power from one spool to another. The required direction and magnitude of the power transfer depends on the engine design and flight cycle conditions.
This power transfer can be achieved by mechanical gearing, but this brings associated problems of weight, reliability and operational flexibility. U.S. Pat. No. 7,791,235 proposes a variable magnetic coupling for transferring torque between the shafts of an aero gas turbine engine.
As well as providing thrust for powering the flight of the aircraft, an aero gas turbine engine generally also has to power an electrical generator for the aircraft's electrical power systems, and to provide mechanical power for a number of accessories such as fuel and oil pumps. It is thus usual to extract power for such accessories from the engine's high pressure spool (or sometimes the intermediate pressure spool) via a gearing and drive shaft arrangement. An advantage of using the high pressure spool for this purpose is that the relatively narrow rotational speed range of the spool is compatible with the operational requirements of electrical generators.
Increased demand for extracted power for accessories can cause problems for engine operability when an engine is operating at or near idle, for example at top of descent or hold.
One option is to extract a fraction of the power from a lower pressure spool, but the wider rotational speed range of such a spool may not be compatible with the requirements of electrical power generation, and furthermore a fixed gear ratio between the spool and its associated generator results in the mass of the generator being significantly increased by the wider speed range of the lower speed spool(s).